1. Field of the Invention
The present invention is directed to a magnetic resonance tomography apparatus as well as to a method for operating a magnetic resonance tomography apparatus.
2. Description of the Prior Art
By means of a radio-frequency field in the megahertz range and a spatially variable magnetic constant field, magnetic resonance tomography utilizes the sharp resonance absorption of magnetic nuclei in biological tissue in order to spatially allocate the nuclear magnetization and display an image of the biological tissue, for example of a human. Due to the spatially variable magnetic constant field, known as the gradient field, the nuclear spin resonant frequency or Larmor frequency of the respective nuclei, particularly hydrogen nuclei, varies spatially. The excited spatial volume of the specimen is thus frequency-coded. The image generation then ensues with a computer on the basis of a Fourier transformation of the raw data which are thus obtained.
A short measuring time is required for avoiding motion artifacts for many applications wherein a movement on the part of the patient is unavoidable, for example in the abdominal or chest region of a patient. Moreover, short examination durations increase the acceptance of magnetic resonance tomography by the patient as well as, due to a higher patient throughput, the economic feasibility of the method. Magnetic resonance methods therefore have been developed with a short measuring time, for example the FISP method, this acronym standing for Fast Imaging with Steady Precession. A pulse sequence composed of an RF excitation pulse and magnetic field gradient pulses in three spatial directions with a repetition time TR is thereby generated that is significantly shorter than the relaxation time T1 and T2 of the nuclear magnetizations of the examination subject. After a transient response whose duration lies on the order of magnitude of the longitudinal relaxation T1 or the transverse relaxation T2, a driven steady state is achieved wherein the nuclear magnetization angle oscillates between two values xc2x1xcex1. An example of such a FISP pulse sequence is disclosed in German PS 44 27 497.
A similar method is described in German PS 35 04 734, which discloses a method and an apparatus for the implementation of the method for the registration of spin resonance data for a spatially resolved examination of a subject that contains nuclear spins and is arranged in a constant, uniform magnetic field that aligns the spins in the field direction. Pulse sequences composed of an RF excitation pulse and a magnetic field gradient pulse are thereby employed. The Rf excitation pulse has a flip angle of significantly less than 90xc2x0, so that only a fraction of the spins aligned by the constant magnetic field are flipped, these, however, being sufficient for generating an echo due to the magnetic field gradient pulse. A steady state already occurs after a few sequences as a result of the T1 relaxation. The sequences are thereby repeated at short intervals, so that extremely short repetition times are achieved. Due to the dynamic steady state, the described method is particularly suited for unlimited, continuous imaging.
Given the above-described methods, the short pulse repetition time enables a fast data acquisition, and thus short measuring times. The disadvantage of these methods, however, is that, in the steady state, the magnetization signal is only dependent on the ratio T1/T2 according to the equation   M  =            M      o        ⁢                  sin        ⁢                  xe2x80x83                ⁢        α                    1        +                              T            1                    /                      T            2                          +                              (                          1              -                                                T                  1                                /                                  T                  2                                                      )                    ⁢          cos          ⁢                      xe2x80x83                    ⁢          α                    
(see H. Morneburg, xe2x80x9cBildgebende Systeme fxc3xcr die medizinische Diagnostikxe2x80x9d, Erlangen 1995, p. 560). With the exception of liquids like blood, gall or liquor in the tissue, however, the quotient T1/T2 is usually relatively constant, so that a good, clinically significant contrast cannot be displayed.
It is an object of the present invention to provide a magnetic resonance imaging method that enables the display of physiologically significant tissue contrasts while still having an optimally short measuring time at the same time.
This object is achieved in a magnetic resonance imaging method wherein successive pulse sequences with an RF excitation pulse and magnetic field gradient pulses are repeated for complete rephasing of the nuclear magnetization, with the pulse generation being interrupted after a measuring cycle containing a defined number of successive pulse sequences before a driven steady state of the nuclear magnetizations is reached and is restarted later. The magnetic resonance signal acquired during the transient time produces a better tissue contrast than a signal acquired in the steady state. Because the measurement is stopped (interrupted) before the steady state is reached and is then restarted, the advantages of a short measuring time can nonetheless be realized.
A pause for relaxation of the nuclear magnetizations can be provided in the thermal equilibrium state. A preparation pulse sequence for preparation of the examination subject can be provided before the beginning of a measurement cycle dependent on the field of application. Preparation methods such as a fat saturation method, an inversion recovery method, a saturation pulse method, a driven equilibrium Fourier transform method or a diffusion pulse method can be used.
For further shortening of the measuring time, measurements in other slice sections can be implemented during a measuring pause in one slice section of the examination subject.
In the inventive method, three through ten pulse sequences are preferably emitted in succession, a measuring pause then ensuing.
The invention is also directed to a nuclear magnetic resonance tomography apparatus with an RF stage for generating RF pulses and a magnet system for generating a constant magnetic field and a gradient magnetic field superimposed thereon, whereby the RF stage and the magnet system being fashioned for generating pulse sequences according to the inventive method described above.